Dose-effect relations and, specifically, cell survival curves are surveyed with emphasis on the interplay of the random factors--biological variability, stochastic reaction of the cell, and the statistics of energy deposition--that co-determine their shape. The global parameters mean inactivation dose, D, and coefficient of variance, V, represent this interplay better than conventional parameters. Mechanisms such as lesion interaction, misrepair, repair overload, or repair depletion have been invoked to explain sigmoid dose dependencies, but these notions are partly synonymous and are largely undistinguishable on the basis of observed dose dependencies. All dose dependencies reflect, to varying degree, the microdosimetric fluctuations of energy deposition, and these have certain implications, e.g. the linearity of the dose dependence at small doses, that apply regardless of unresolved molecular mechanisms of cellular radiation action.